1. Field of the Invention
The invention relates generally to a device and method for a sensor. More particularly, the present invention relates to a method for producing a Schottky barrier diode that is particularly suited to serve as an ultra-violet sensor.
2. Background Description
Spacecraft and other vehicles, such as satellites, need to successfully operate in the presence of a wide range of radiation wavelengths sometimes at extreme ultraviolet (EUV) wavelengths. For example, the Sun has a spectrum that is dominated by a few bright emission lines. The intensity of the solar EUV flux is highly variable, often changing by a factor of two in a period of one hour and by an order of magnitude from solar minimum to solar maximum. Absorption of this variable solar EUV radiation by the Earth's upper atmosphere can disrupt RF communications, causes errors in GPS navigation, and increases the drag on satellites in low-Earth orbits. To aid in characterizing these effects, it is advantageous to have an Extreme Ultra Violet Sensor (EUVS) that will measure the total solar flux in a number of EUV spectral bands.
Present devices serving as EUVS consist of single large-area (about 1 cm2) silicon photo-diode detectors and may be employed in an apparatus that senses radiation. Typical radiation sensing devices may have at least five channels, some of which operate in conjunction with telescopes. One channel may operate at the Lyman-a wavelength of 122 nm, one of the longest wavelengths serviced by the EUVS. This channel may have a spectral filter that comprises a thin metallic film deposited on a window and a single detector operating in the first order beam of a grating associated with the respective telescope.
Each photo-diode detector serving as an EUVS and operating in the five-channel apparatus has a Woodes filter consisting of a thin metallic film deposited directly on the front surface. In the EUV band, the threshold for detection (the spectral irradiance level at which the signal to noise ratio must exceed unity) is about 1-2 mW/m2-nm. The total solar irradiance, integrated over all wavelengths, is about 1370 W/m2. Of this total irradiance, about 1000 W/m2 occurs in the wavelengths range from approximately 200-100 nm from the near UV to the band-gap of silicon. In order to maximize the transmission of the filter within the spectral band, present devices include a channel design with a thin film of metal such that the transmissivity is peaked within the spectral band. These devices are often not suitable for use with EUV due to problems with long-term instability and inadequate rejection of visible light. The formation of inter-metallic compounds probably contributed to both of these problems as the detector current, due to dark and long-wave radiation, greatly exceeds the “signal” current produced by in-band EUV radiation.
Further, the signal-to-noise ratios (SNRs) of the signal current do not satisfy the EUVS requirements and, therefore, improved EUVS need to be provided. Enhanced requirements place greater demands upon the system to reduce detector dark current and current generated by visible radiation.
Thus, there is a need for an improved EUVS that can isolate small vacuum ultraviolet signals from the large, variable visible-near-infra-red signal background. The improved EUVS should take into account the fact that the primary source of out-of-band photo-current is radiation for which the photon energy exceeds the band-gap of the typical detector, since only these photons are capable of producing photo-current in the typical detector. For example, for silicon photo-detectors, this spectral region includes all wavelengths shorter than the silicon cut-off wavelength of approximately 1100 nm. The typical design goal for the filters is a transmissivity of 0.1-0.3 in the EUV band and <10-5 in the “visible” band from 200-1100 nm. The EUV radiation that is diffracted by the transmission grating onto the detector contains not only the first diffraction order, but also its harmonics. Therefore, it is critical for the filters to have low transmission values at these harmonic wavelengths.